Implement galaxy_infrastructure_url config option...

Allows Galaxy to know how to specify other components should communicate with its API. Used immediately as a reasonable default for Pulsar MQ mode and will be used later by the Docker IPython stuff for communicating with the API and for this card https://trello.com/c/hL4NYsSp.
Update ie.mako to use galaxy_infrastructure_url if available...

Also use the method guess_galaxy_port on Galaxy's config object that was moved there from ie.mako in a previous Galaxy commit.